134 Papers from the Marine Biological Laboratory at Tortugas. 



of the body from the anterior end. Between these bands the body now begins 

 to be somewhat concave. In general appearance at this time the larva 

 resembles that of Asterias more than it does that of an echinoid. At 66 hours 

 the bands are well developed and the region between the bands has become 

 still more concave. 



Between this time and 72 hours a further increase in size and change 

 in form of the body is apparent, the posterior half becoming much broader 

 and the posterior end more rounded, while by reason of this change of form 

 the anal region has been shifted forward. During the period from 60 to 65 

 hours the oesophagus, stomach, and intestine differentiate from the archen- 

 teron. About 72 hours after fertilization the first indication of a larval 

 skeletal system may be noted in the triradiate spicules arising in groups of 



Fig. 7. — Toxopneustes pluteus. 6 days. 



mesenchyme cells in the broadest part of the body, about the level of the 

 anterior half of the stomach, and just beneath the postoral band of cilia. 

 The laterally directed axis is seen to be double and gives rise to a rod of 

 the latticed type (fig. 5). During the next day the body continues to 

 increase in size, the anterior and posterior ciliated bands become connected 

 about the edges of the ventral depression, and the postero-lateral (?) arms 

 begin to push out laterally from the body. A short medio-dorsal rod may 

 be seen developing in the skeletal system (fig. 6). 



A brief inspection of a figure of a Toxopneustes pluteus of the same age, 

 6 days (fig. 7), is sufficient to convince one of the striking difference in form. 

 It is evident from a comparison of the development of these two forms that 

 if a cross between the two were possible valuable data might be afforded. 



